1. Field of the Invention
The present invention is directed generally to optical scanning device that has great flexibility in layout design of scanning optical system and narrow variation in beam-spot size, and is directed to image forming apparatus that uses the optical scanning device.
2. Description of the Related Art
In a typical optical scanning device for use in a laser printer, an optical plotter, a digital copying machine, a facsimile apparatus, or the like, optical scanning is performed such that a light beam from a light source device is caused to strike an optical deflector, such as a polygon mirror, to be deflected thereby and thereafter to travel through a scanning optical system arranged downstream in optical path of the optical deflector. The light beam is focused through the scanning optical system onto a scanning surface, which is the surface of a photoconductive, on a photosensitive image carrier.
A specific example of such an optical scanning device includes a type that includes four image carriers such as photosensitive drums aligned in a conveying direction of a transfer sheet, and that is configured such that each of the surfaces of the image carriers is optically scanned. Light beams, emitted from a plurality of light source devices corresponding to these image carriers, are deflected by a single optical deflector so as to simultaneously illuminate the image carriers for exposure through a plurality of scanning optical systems, each corresponding to one of the image carriers; hence, a latent image is formed on each of the image carriers. These latent images are developed with developing agent of different colors, such as yellow, magenta, cyan, and black, into visible images which are then transferred onto a single transfer sheet to be superimposed on one another. Thus, a multiple-color image is formed.
To meet recent demands for space-saving design of image forming apparatuses, demands have also arisen for compact, low-profile design of optical scanning devices used in the image forming apparatuses. Meanwhile, optical scanning devices are required to be of not only compact, low-profile design but also to be enhanced in image quality. To enhance image quality, it is necessary to reduce beam-spot size and prevent a range of variation in beam-spot size from becoming wide.
Layout design of a reflecting mirror, which is arranged such that light having been deflected by an optical deflector is incident on the reflecting mirror, is a key factor for compact design of optical scanning devices; by arranging a scanning lens close to the optical deflector, flexibility in layout design of the reflecting mirror can be increased.
To reduce size of beam spot on a scanning surface and variation in beam-spot size, it is required to perform compensation related to various optical properties, such as field curvature, wavefront aberration, and magnification error. To perform the compensation related to these properties, a plurality of scanning lenses are preferably used.
Examples of a scanning optical system that includes a plurality of scanning lenses arranged near an optical deflector include an optical scanning device disclosed in Japanese Patent Application Laid-open No. 2001-296491.
The optical scanning device disclosed in Japanese Patent Application Laid-open No. 2001-296491 is configured such that the distance between an optical deflector and a scanning surface is relatively short, a plurality of scanning lenses are arranged near the optical deflector, and a field curvature in main-scanning cross section is compensated depending on surface geometry of the scanning lens. As for the main-scanning direction, magnification of an optical system substantially depends on a focal distance of a coupling lens; accordingly, field curvature can be compensated by adjusting geometry of the scanning lenses, thereby narrowing variation in beam-spot size. As for the sub-scanning direction, sub-scanning lateral magnification depends on layout design of and power distribution among the scanning lenses. By adopting such configuration as described above, flexibility in layout design of reflecting mirror is increased.
However, positions and geometry of the lenses in the optical scanning device disclosed in Japanese Patent Application Laid-open No. 2001-296491 are disadvantageous in involving considerably large sub-scanning lateral magnification. For example, according to layout design of the first embodiment described in Japanese Patent Application Laid-open No. 2001-296491, lateral magnification is −6.68 times in the sub-scanning direction. As the sub-scanning lateral magnification increases, the magnitude of influence exerted by dimensional variation and variation in installation positions of the optical deflector and the scanning lenses increases; accordingly, even when field curvature and magnification error are favorably compensated in theory, an actual product exhibits relatively wide variation in beam-spot size. This disadvantage can be prevented by achieving high precision in dimensional accuracy, processing accuracy, and accuracy in installation positions of polygon mirror and optical system upstream, in optical path, of the optical deflector; however, this countermeasure can lead to additional manufacturing cost.
To this end, a technique is disclosed in Japanese Patent Application Laid-open No. 2004-086019 related to an optical scanning device for reducing sub-scanning lateral magnification of scanning optical system to approximately −1× by arranging a second scanning lens near an endpoint of an optical path through which light travels from a polygon mirror to a scanning surface.
Meanwhile, for a scanning optical system, use of a plurality of lenses is generally advantageous to compensation for field curvature and wavefront aberration and therefore leads to reduction in beam-spot size. However, use of a plurality of scanning lenses undesirably reduces flexibility in layout design of a reflecting mirror, which is disclosed in an optical scanning device configured according to the technique disclosed in Japanese Patent Application Laid-open No. 2004-086019. Particularly in a color laser printer or the like that includes reflecting mirrors for four stations, flexibility in layout design of the reflecting mirrors is much reduced, which is disadvantageous.
Because arranging the reflecting mirror downstream of the second scanning lens can shorten distance between mirrors, particularly in an optical system that includes two or three reflecting mirrors, this can disadvantageously make it difficult to ensure space for use in adjusting layout design of the reflecting mirrors. Optical scanning devices, configured to provide a wide variety of choices of layout design of reflecting mirrors, are less susceptible to this disadvantage. However, this disadvantage has a large effect on the optical scanning device configured as disclosed in Japanese Patent Application Laid-open No. 2004-086019, because the optical scanning device has reduced flexibility in layout design of the reflecting mirrors. The reflecting mirror can be positioned between the two scanning lenses rather than downstream from the second scanning lens; however, this arrangement is disadvantageous in that light beam can obliquely fall on the second scanning lens, resulting in curved scanning line or the like. To position the second scanning lens downstream from the reflecting mirror, at which scan width is relatively large, it is necessary to employ a lens of a greater length as the second scanning lens; this can lead to additional manufacturing cost.
As described above, arranging the scanning lens near the optical deflector with higher priority on flexibility in layout design of the reflecting mirror can increase the sub-scanning lateral magnification and widen variation in beam-spot size. On the other hand, disadvantageously can reduce flexibility in the layout design of the reflecting mirror, when arranging the second scanning lens near the endpoint of the optical path through which light beam travels from the optical deflector to the scanning surface to place higher priority on reduction in sub-scanning-direction lateral magnification.